surrogate key

A unique primary key generated by the RDBMS
that is not derived from any data in the database and whose
only significance is to act as the primary key.

A surrogate key is frequently a sequential number (e.g. a
Sybase "identity column") but doesn't have to be. Having
the key independent of all other columns insulates the
database relationships from changes in data values or database
design and guarantees uniqueness.

Some database designers use surrogate keys religiously
regardless of the suitability of other candidate keys.
However, if a good key already exists, the addition of a
surrogate key will merely slow down access, particularly if it
is indexed.